The typical class 8 truck driveline uses a transmission physically attached, such as by bolting, to the engine. A tandem drive axle utilizing an inter-axle differential is connected to the transmission with a drive axle.
The engine/transmission assembly is typically mounted on rubber-type bushings. Usually, three bushings are utilized with one bushing located at the front of the engine and the other two bushings on each side of the clutch bell housing. This arrangement requires a compromise in the engine mounting bushing stiffness since the transmission multiplied torque requires stiff bushings to support the large torque reaction but a softer bushing would reduce noise, vibration and harshness (NVH) from the engine. There is also a trend toward lower speed, higher torque engines, which will increase the loads on the engine mounts and again pushes designers toward stiffer bushings that do little for NVH. Finally, there is a need to improve the ride of class 8 trucks by reducing the tandem axle unsprung weight.
Unsprung weight as used herein is the weight of the suspension, wheels, tires and other components directly connected to them, rather than supported by the suspension. Sprung weight at used herein is the weight of the vehicle body and other components supported by the suspension.
Noise, vibration and harshness can be improved by decoupling the engine and transmission. This has the advantage of being able to mount the engine on bushings specifically tuned for the engine vibration modes without the transmission torque multiplication. A transmission mounted remotely from the engine on the vehicle frame could also utilize tuned bushing mounts for the transmission vibration modes. This remote transmission location could incorporate part of the tandem axle components, such as the inter-axle differential. This would reduce the tandem unsprung weight for improved ride characteristics. Since the remote mounted transmission is more difficult to shift manually, an automated transmission shift control could be used. To further reduce the tandem axle unsprung weight, the tandem axle wheel differentials could be integrated into the frame mounted transaxle assembly and a De Dion type axle and suspension system utilized or a fully independent suspension system used in conjunction with the transaxle unit.